The present invention relates to a method for the production of paper and board, wherein there is used as a retention aid in the retention system a solution of a cationic polymer together with a microparticle mixture which contains a swellable clay of the smectite group.
At present, the use of microparticles in the retention system of paper production, in particular in the production of fine paper, is very common, the aim being to improve further the efficiency of the production process. The advantages of the adoption into use of microparticles include improved retention, more efficient dewatering, and better formation. The most effective of the microparticles in use are colloidal silica-based microparticles of various types, solid or sol, and bentonite-like swellable natural materials belonging to the smectite group of clays. Instead of, or in addition to, a microparticulate compound it is possible to use as a retention aid in the retention system polymers, which may be anionic, cationic or non-ionic, and which are characterized by a high molecular weight. The problem involved with these compounds is typically excessive flocculation, which deteriorates the optical properties of paper.
The silicates may be natural crystalline minerals or synthetic materials. Synthetic silicates have the advantage of better controllable properties, in which case the efficiency of the microparticulate material used can be maximized. The colloidal synthetic silicates used as retention aids in retention systems include, for example, colloidal silica and polysilicate, aluminum silicates, and aluminum silicates modified with alkali metals and with alkaline-earth metals. The particle size of these materials is typically a few nanometers or a few tens of nanometers, and they are more expensive than, for example, bentonite.
The minerals of the smectite group of natural clays include montmorillonite, beidellite, nontronite, saponite and sauconite, which are composed mainly of aluminum silicates and some of which contain, in addition to sodium, also other cations, such as magnesium, iron, calcium or zinc. Smectites also include hectorite and vermiculite, which are, instead, composed mainly of magnesium silicate and contain to a lesser extent also other cations. Natural clays are typically somewhat darker than synthetic materials, owing to impurities present in them.
Bentonite is a species of rock mainly composed of montmorillonite (Kirk-Othmer Encyclopedia of Chemical Technology, Part 6, 4th edition, p. 394). However, the name bentonite is commonly also used of commercial products which contain mainly montmorillonite. Bentonite-type materials have been used in paper production especially as materials adsorbing impurities. Natural hectorite is mainly composed of magnesium silicate. In hectorite, some of the exchangeable sodium ions have been replaced by lithium ions. In addition the structure contains some fluoride.
Bentonite has been used as a retention aid in paper production together with a cationic polymer in the patent U.S. Pat. No. 4,753,710 of Allied Colloids. In the process according to the patent, a cationic polymer, preferably polyethylene imine, a polyamine epichlorohydrin product, a polymer of diallyl dimethyl arrmonium chloride, or a polymer of acrylic monomers, was added to an aqueous cellulosic suspension before the last shearing stage, and bentonite was added after this shearing stage. Improved retention, dewatering, drying, and web forming properties were thereby achieved. In the microparticle system according to the method there is used bentonite, which is available under the trade name HYDROCOL.
Respectively, in the paper production method according to the patent U.S. Pat. No. 5,178,730 of Delta Chemicals, there is added to the pulp before the shearing stage a cationic polymer, which is preferably a tertiary or quaternary amine derivative of polyacrylamide, and after the shearing stage, before the headbox, there is added a natural hectorite at a weight ratio of 0.5:1-10:1. It has been observed that the combination of polymer and hectorite used in the method affects filler retention and dewatering more effectively than does, for example, bentonite used in a corresponding manner. The method according to the patent can be used in both alkaline and acid paper production recipes.
In the patent U.S. Pat. No. 5,876,563 of Allied Colloids, a cationic starch together with a cationic polymer and an anionic microparticulate material is used as the retention aid. The microparticulate material suggested for use in this connection is, for example, bentonite or colloidal silica or polysilicate microgels or polysilicic acid microgels together with aluminum-modified colloidal silica, or aluminum-modified polysilicate mnicrogel or aluminum-modified polysilicic acid microgel, of which a suspension is formed.
In the application WO 99/14432 of Allied Colloids, the microparticulate aid is preferably bentonite, colloidal silica, polysilicic acid, polysilicate microgel, or an aluminum-modified version thereof.
In Finnish patents 67735 and 67736, there is used, together with a hydrophobic size, a retention aid combination which contains, together with a polymer, preferably polyacrylamide, as an anionic component a colloidal silicic acid, bentonite, carboxymethyl cellulose or carboxylated polyacrylamide.
The use of silicate microparticles together with a cationic polymer in a retention system is described in the patent U.S. Pat. No. 5,194,120 of Delta Chemicals. The prevalent cation in the synthetic amorphous metal silicate was Mg, and the polymer was preferably a ternary or quaternary amine derivative of polyacrylamide, their weight ratio being between 0.03:1 and 30:1. By the method, retention, dewatering and formation were improved by using smaller amounts of retention aids than previously, and thus the costs were correspondingly lower.
According to our observations, when bentonite is used together with polyacrylamide, it serves as an effective microparticulate material in the retention system. Compared with this, a synthetic metal silicate in which the prevalent cation is Mg is, in a corresponding situation, not as effective as bentonite.
We have observed, surprisingly, that when there is used a microparticle mixture in which the major part consists of bentonite or hectorite and to which a small amount of a synthetic metal silicate having magnesium as the prevalent cation is added, the said mixture serves as a microparticulate material more effectively than does either component of the mixture, bentonite or hectorite or synthetic metal silicate, separately.
According to the invention there is thus provided a method for producing paper or board in such a manner that retention aids are added to the stock stream passing to the paper machine headbox, the stock stream is directed to the wire, the stock is dewatered in order to form a paper web, and the paper web is dried, the method being characterized in that the retention aids used are a solution of a water-soluble cationic polymer and a microparticle mixture which contains, in the form of a suspension, a swellable clay of the smectite group and a colloidal synthetic metal silicate, the prevalent cation in the synthetic metal silicate in the suspension being magnesium.
The said swellable clay of the smectite group, hereinafter in the specification referred to as clay material, is preferably bentonite or hectorite.
The microparticle mixture in the form of a suspension is preferably prepared by mixing the said clay material, preferably bentonite or hectorite, and the said metal silicate together while dry. A suspension is made from the dry mixture by slurrying the dry mixture in water, preferably to a concentration of 1-20%, and especially preferably to a concentration of approx. 5%.
The microparticle mixture can be transported and stored in the form of a suspension, but preferably the microparticle mixture is transported and stored in a dry form, and a suspension is prepared from it on site, immediately before use.
The proportion of the clay material in the microparticle mixture may be 85-99% by weight and that of the metal silicate 1-15% by weight. Preferably the mixing ratio of the synthetic metal silicate to the clay material is 0.03-0.1. The total amount of the microparticle mixture to be added to the stock is preferably at minimum 0.05%, especially preferably 0.1-0.25%, of the dry solids weight of the stock.
According to the invention, the retention aids are preferably added in steps so that first the solution of a cationic polymer is added, whereafter there follows a shearing process step for breaking up flocs, and thereafter the microparticle mixture in suspension form is added.
By the use of the microparticle mixture according to the invention, a surprisingly good retention is achieved, although when the clay material or the synthetic metal silicate is used alone as a retention aid, the retention result remains poorer. It can be assumed that the synergy advantage is based on the ability of the simultaneously added silicate to promote a more uniform distribution of the clay material particles into the aqueous phase, whereupon the surface area of the clay material particles can be exploited more effectively. When the microparticle mixture according to the invention is used as a retention aid, the filler retention may be up to 5 percentage points better than when the individual components of the mixture are used in the same amounts of dosage. A similar result is obtained for the total retention, even though the change is not as clearly observable as regarding the filler retention, since filler constitutes most of the stock fraction more difficult to retain on the wire.
The reproducibility of the measuring results is especially significant; without exception, a better retention result is always obtained with the mixture, regardless of the prodiiction conditions, than with the individual components of the mixture.
Furthermore, the color of the microparticle mixture is somewhat lighter than that of pure bentonite.
By the use of the microparticle mixture according to the invention a high retention is attained by using a smaller amount of retention aid as compared to the use of the individual components of the mixture. In this case, for example, dust problems and the consequent handling problems are smaller. The efficiency ratio of the use of microparticles is improved as the attained efficiency can be maintained constant and the amount of material to be added can be reduced.
The synthetic metal silicate according to the invention must have a sufficiently high and preferably controllable cation exchange capacity. Typically the exchangeable cation may be, for example, Li+. The prevalent cation is magnesium, as in, for example, the product sold under the trade name of Laponite. The clay material may be any commercial bentonite or bentonite-type material, such as montmorillonite, beidellite, nontronite, saponite, sauconite, vermiculite or hectorite, or a chemically modified version of these. Advantageously bentonite can be used, for example, the Kemira Chemicals product sold under the trade name of Altonit SF or natural hectorite.
The cationic polymer used in the invention can be produced advantageously by copolymerizing acrylamide with a cationic monomer or methacrylamide with a cationic monomer. The molecular weight of the cationic polymer is preferably at least 500,000, and it is added to the stock preferably in an amount of at minimum 0.02%, especially preferably 0.03-0.05%, of the dry solids weight of the stock.
The cationic polymer used in the invention may be any copolymer of acrylamide and/or methacrylamide, prepared using at least as one of the comonomers a cationiccally charged or cationically chargeable monomer. Such monomers include methacryloyloxyethyltrimethyl ammonium chloride, acryloyloxyethyltrimethyl ammonium chloride, 3-(methacrylamido)propyltrimethyl ammonium chloride, 3-(acryloylamido)propyltrimethyl ammonium chloride, diallyldimethyl ammonium chloride, dimethylaminoethyl acrylate, dimethylaminoethyl methacrylate, dimethylaminopropylacrylamide, dimethylaminopropylmethacrylamide, or a similar monomer. The polymer may also contain monomers other than acrylamide, methacrylamide, or some cationic or cationizable monomer.
The cationic polymer may also be a polymer which has been treated afterwards to render it cationic, for example, a polymer prepared from polyacrylamide or polymethacrylamide by using Hofmann or Mannich reactions.
The cationic polymer may be prepared by conventional radical-initiation polymerization methods, and as a product it may be either dry powder or an emulsion of a polymer solution in an organic medium.
Before dosing, preferably an 0.05-0.5% solution, especially preferably an 0.1-0.3% solution, is prepared of the polymer, which solution may be further diluted before the feeding point in order to ensure good mixing.
The method according to the invention was observed to be robust with respect to various test arrangements, pulps, and fillers. The stock material and its initial pulp may, for example, be composed of a conventional chemical pulp or mechanical pulp or of other conventional raw materials used in paper making, such as recycled paper. The filler, which may be, for example, ground or precipitated calcium carbonate, kaolin, calcined kaolin, talc, titanium dioxide, gypsum, synthetic inorganic or organic filler, preferably, however, calcium carbonate, is incorporated into the pulp by a conventional method before the adding of the cationic polymer. The method according to the invention can be used in any conventional paper- or board-making apparatus. Furthermore, the method is not critical as regards the effect of the synthetic metal silicate type or of the mixing ratio of bentonite and metal silicate.
By the method according to the invention, retention can be improved further as compared with prior known methods and, at the same time, if so desired, the amount of the required retention aid can be reduced, whereupon any detrimental effects caused by its use are slighter.
The invention and its embodiments are described below with the help of various examples; the purpose of the examples is, however, not to restrict the scope of the invention.